I adapted the example on the following page to show you a quick test:

http://help.adobe.com/en_US/ActionScript/3.0_UsingComponentsAS3/WS5b3ccc516d4fbf351e63e3d118a9c65b32-7f4a.html

I changed the drawLayout function to highlight the word "renderer" in red. You need to import TextFormat and TextField classes too.

override protected function drawLayout():void {
  textField.width = this.width;
  var text:String = textField.text;
  var tf:TextFormat = textField.getTextFormat();
  var redIdx:int = text.indexOf("renderer");
  if (-1 != redIdx) {
    tf.color = 0xff0000;
  }
  TextField(textField).setTextFormat(tf, redIdx, (redIdx + 8));
  super.drawLayout();
}

Try that...